sound: Simplify getchns()

Remove all special handling for SIMPLEX, since we can just fetch the
channel directly.

While here:

• Get rid of a no-op getchns() call in dsp_ioctl().
• Rename getchns() to dsp_lock_chans(), and relchns() to dsp_unlock_chans().
• Simplify DSP_FIXUP_ERROR(), as we do not longer assign SD_F_PRIO* flags to the softc.

Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: dev_submerge.ch
Differential Revision: https://reviews.freebsd.org/D45775